University  of  Texas  Bulletin 

No.  1822:     April  15,  1918 


The  Anticlinal  Theory  as  Applied  to  Some 
Quicksilver  Deposits 


BY 
J.  A.  UDDEN 


Bureau  of  Economic  Geology  and  Technology 

Division  of  Economic  Geology 
J.  A.  Udden,  Director  of  the  Bureau  and  Head  of  the  Division 


PUBLISHED  BY 
THE  UNIVERSITY  OF  TEXAS 

AUSTIN 


Publications  of  the  University  of  Texas 

Publications  Committee : 

F.  W.  GRAFF  R.  H.  GRIFFITH 

J.  M.  BRYANT  J.  L.  HENDERSON 
D.  B.  CASTEEL  I.  P.  HILDEBRAND 
FREDERIC  DUNCALF  E.  J.  MATHEWS 

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University  of  Texas  Bulletin 

No.  1822:     April  15,  1918 


The  Anticlinal  Theory  as  Applied  to  Some 
Quicksilver  Deposits 


BY 


J.  A.  UDDEN 

« < 


Bureau  of  Economic  Geology  and  Technology 

Division  of  Economic  Geology 
J.  A.  Udden,  Director  of  the  BureaiTand  Head  of  the  Division 


PUBLISHED  BY  THE  UNIVERSITY  SIX  TIMES  A  MONTH.  AND  ENTERED  AS 

SECOND-CLASS  MATTER  AT  THE  POSTOFFICE  AT  AUSTIN,  TEXAS. 

UNDER  THE  ACT  OF  AUGUST  24.  1912 


•  •; 


The  benefits  of  education  and  of 
useful  knowledge,  generally  diffused 
through  a  community,  are  essential 
to  the  preservation  of  a  free  govern- 
ment. 

Sam  Houston 


Cultivated  mind  is  the  guardian 
genius  of  democracy.  .  .  .  It  is  the 
only  dictator  that  freemen  acknowl- 
edge and  the  only  security  that  free- 
men desire. 

Mirabeau    B.    Lamar 


EXCHANGE 


CONTENTS 


Introductory 7 

The  W.  K.  Ellis  Mine 11 

The  Marf a  and  Mariposa  Group 14 

Study  Butte 16 

The  Christmas  Mountain  Prospects 18 

The  Workings  on  Section  248 20 

The  Coltrin's  Camp  Prospect 22 

The  Laguna  Float 22 

The  Chisos  Mine 25 

Conclusions  .  .   30 


LIST  OF  ILLUSTRATIONS 

Fig.  1.  Sketch  map  showing  localities  of  quicksilver  ore  described 
in  the  text 8 

Fig.  2.  Map  of  Mariscal  Mountain  showing  approximate  position 
of  the  three  sections  in  Fig.  3 10 

Fig.  3.  Diagrammatic  sections  from  east  to  west  across  the  Mar- 
iscal Mountain.  Positions  of  sections  shown  in  Fig.  2 12 

Fig.  4.  Sketch  map  of  the  vicinity  of  the  Marfa  and  Mariposa 
and  the  Old  Terlingua  Mines  showing  the  position  of  section  given 
in  Fig.  5 14 

Fig.  5.  Diagrammatic  section  extending  from  south-southwest  to 
north-northeast  along  the  line  indicated  in  Fig.  4.  Known  occur- 
rences of  ore-bearing  rock  are  approximately  indicated  by  solid 
black 15 

Fig.  6.  Sketch  map  of  Study  Butte  showing  the  position  of  the 
section  in  Fig.  7 17 

Fig.  7.  Section  of  Study  Butte  extending  from  north  to  south 
along  the  line  indicated  in  Fig.  6.  Known  occurrences  of  cinnabar- 
bearing  rock  roughly  indicated  by  solid  black 18 

Fig.  8.  Map  of  Crristmas  Mountain  showing  approximate  posi- 
tion of  section  in  Fig.  9 19 

Fig.  9.  Diagrammatic  section  from  southwest  to  northeast  across 
the  Christmas  Mountain  along  line  indicated  in  Fig.  8.  Known  cin- 
nabar-occurrences indicated  by  solid  black 20 

Fig.  10.  Map  of  Section  248,  and  parts  of  sections  247,  230,  and 
229,  Block  G-4,  west  of  the  Chisos  Mountains,  Brewster  County,  show- 
ing location  of  section  a-b,  in  Fig.  11 21 

Fig.  11.  Diagrammatic  section  along  the  line  a-b  in  Fig.  10. 
Known  occurrences  of  cinnabar  are  indicated  by  solid  black ....  22 

Fig.  12.  Sketch  map  of  the  Chisos  Mountains  showing  position 
of  the  section  in  Fig.  13 23 

Fig.  13.  Diagrammatic  section  across  the  Chisos  Mountains  from 
west-southwest  to  east-northeast  on  the  line  indicated  in  Fig.  12  24 

Fig.  14.  Map  of  the  region  around  the  Chisos  Mining  Company 
property  showing  the  Coltrin's  camp  and  the  course  of  the  section 
a-b  given  in  Fig.  15  and  of  section  c-d  in  Fig.  16 26 

Fig.  15.  Diagrammatic  section  running  approximately  from  north 
to  south  along  the  line  a-b  indicated  in  Fig.  14.  Known  localities 
of  ore-bearing  rock  indicated  by  solid  black 27 

Fig.  16.     Generalized  section  along  the  line  c-d  in  Fig.  24. ...     29 


THE  ANTICLINAL  THEORY  AS  APPLIED  TO  SOME 
QUICKSILVER  DEPOSITS* 


J.  A.  UDDEN 
INTRODUCTORY 


From  his  general  studies  of  the  quicksilver  deposits  of  tho 
world,  Becker  came  to  the  conclusion  that  the  source  of 
quicksilver  is  to  be  found  in  deep-lying  crystalline  rocks, 
such  as  granites.  On  such  an  hypothesis  the  distribution 
of  quicksilver  in  the  Terlingua  district  also  would  seem  to 
have  a  natural  explanation.  Cinnabar  is  here  found  in 
quite  diverse  kinds  of  rocks,  ranging  from  the  Edwards 
limestone  in  the  Comanchean  to  the  sandstones  of  the  Rat- 
tlesnake beds  in  the  Upper  Cretaceous.  It  also  occurs  in 
some  igneous  rocks  which  intrude  these  sediments.  It  has 
even  been  reported  from  Paleozoic  rocks  in  the  Solitario 
uplift,  though  the  present  writer  has  no  reliable  information 
on  this  occurrence.  Under  all  circumstances  it  would  seem 
that  the  cinnabar  is  not  limited  in  its  occurrence  here  to  any 
particular  formation.  That  the  source  is  deep-seated  may 
also  be  inferred  from  the  fact  that  the  mineral  is  distributed 
over  a  fairly  wide  area  extending  from  the  Mariscal  Moun- 
tains on  the  east  to  the  Lajitas  Mesa  on  the  west,  and  from 
beyond  the  Mexican  boundary  on  the  south  to  the  Christ- 
mas Mountains  on  the  north.  The  known  east  and  west 
extension  of  the  district  is  about  thirty  miles,  and  the 
greatest  north  and  south  extension  is  at  least  twenty  miles. 
It  is  also  interesting  to  note  that  all  the  quicksilver  may 
be  said  to  occur  within  the  limits  of  a  huge  sunken  block 
just  west  of  the  main  Front  Range  of  the  Rock  Moun- 
ains.  The  sinking  of  this  block  must  have  been  accom- 
panied by  a  rising  of  the  isogeothermals  with  reference 
to  the  parts  of  which  this  block  itself  consists.  The  deep- 
seated  heat,  which  may  be  looked  upon  as  having  been  re- 


*Read  before  the  Geological  Society  of  America  at  its  meeting  in 
Baltimore,  December  28,  1918. 


8 


University  of  Texas  Bulletin 


sponsible  for  the  upward  migration  of  the  quicksilver,  is 
here,  as  in  California,  yet  in  evidence  in  hot  springs.  Such 
waters  emerge  just  east  of  the  district  on  the  banks  of  the 
Rio  Grande.  The  fact  that  the  present  mines,  which  repre- 
sent the  best  ore  so  far  found,  are  strung  on  a  line  extending 
from  west-northwest  to  east-southeast,  suggests  the  possible 
existence  of  some  linear  fracture  at  a  depth  sufficient  to  be 
entirely  concealed  by  the  more  superficial  structures  in  the 
"outer  crust"  of  the  earth. 


so, 


Fig.  1.     Sketch  map  showing  localities  of  quicksilver  ore  described  in  text. 

The  date  of  the  deposition  of  the  quicksilver  is  probably 
quite  late,  since  the  deposits  have  been  found  not  only  in 
the  latest  Cretaceous  sediments,  but  also  in  some  of  the 
intrusives  which  in  this  region  are  regarded  as  belonging 
to  the  last  half  of  the  Tertiary  period. 

The  present  writer,  who  has  had  opportunity  to  see  most 
of  the  quicksilver  occurrences  in  Brewster  County,  believes 
that  structural  conditions  clearly  affect  deposition  in  this 
district  and  have  a  direct  practical  bearing  on  the  finding 


Anticlinal  Theory  of  Quicksilver  9 

and  development  of  the  quicksilver  ores.  Briefly  stated 
observations  here  indicate  that  the  deposition  of  the  quick- 
silver fumes  rising  from  great  depths  has  resulted  from  the 
capture  of  the  fumes  and, from  their  retention,  mainly  in 
the  form  of  sulphides,  in  structures  practically  resembling 
those  which  determine  the  retention  of  upwardly  migrating 
liquid  and  volatile  bitumens.  The  quicksilver  deposits  in 
almost  every  case  occur  in  anticlines  and  domes  or  along 
decided  belts  in  what  are  known  as  "structural  terraces,"  or 
arrested  monoclines.  Invariably  they  occur  in  these  struc- 
tures at  levels  where  the  rising  solutions  have  encountered 
strata  that  are  less  pervious  than  those  immediately  below. 
This  appears  to  be  true  both  on  a  large  and  on  a  small  scale. 
The  richest  cinnabar  deposits  so  far  encountered  have  l>een 
found  in  the  contact  between  the  Georgetown  (which  is  the 
upper  fifty  feet  of  a  thick  limestone  usually  referred  to  as 
the  Edwards)  and  the  Del  Rio  clay.  The  next  richest  hori- 
zon has  been  proved  to  be  the  contact  between  the  Buda 
limestone  and  the  overlying,  less  penetrable  flags  and  shales 
of  the  Eagle  Ford.  In  the  Eagle  Ford  itself,  which  in  this 
region  measures  at  least  some  600  feet  in  thickness,  and 
which  consists  of  beds  rapidly  alternating  from  compart 
limestone  to  black  shale,  the  distribution  of  ore  frequently 
shows  a  direct  relation  to  these  alternations.  Even  in  the 
small  details  of  these  beds  it  has  been  found  that  pockets 
of  cinnabar  frequently  underlie  layers  of  shale.  It  may  be 
that  there  is  a  coincidence  of  other  circumstances  affecting 
this  arrangement  in  the  distribution  of  the  cinnabar.  All 
of  these  shales  are  more  or  less  bituminous  and  contain  or- 
ganic matter.  This  may  have  aided  in  the  precipitation  of 
the  quicksilver  fumes  as  sulphides.  But  that  the  structures 
themselves  have  been  the  most  important  factor  is  indicated 
by  the  fact  that  bitumens  also  have  accumulated  in  the  same 
structures.  Oil  has  been  found  in  the  ore  in  the  Eagle  Ford 
in  sufficient  quantity  to  materially  aid  in  its  reduction  in  the 
furnace. 

I  have  on  a  previous  occasion  briefly  described  my  views 


10 


University  of  Texas  Bulletin 


o 


Scale 
i  a. 


3  Miles 


Fig.  2.      Map  of  Mariscal  Mountain  showing  approximate  position  of  the 
three  sections  in  Fig.  3. 


Anticlinal  Theory  of  Quicksilver  11 

on  the  distribution  of  the  quicksilver  in  this  region.*  It  is 
my  purpose  here  to  present  in  slightly  greater  detail  than 
before  the  evidence  which  illustrates  my  views,  some  of 
which  evidence  has  come  to  light  through  more  recent  de- 
velopments of  the  deposits  in  this  region.  I  will  therefore 
briefly  describe  the  natural  conditions  as  they  exist  at  the 
several  points  where  quicksilver  ores  occur,  and  also  ven- 
ture to  present  my  interpretation  of  the  significance  of  the 
conditions  noted. 

The  W.  K.  Ellis  Mine 

The  easternmost  mine  in  this  district  is  the  W.  K.  Ellis 
mine,  located  on  the  north  end  of  the  Mariscal  Mountain. 
This  mountain  is  an  anticline  extending  from  Mexico  across 
the  Rio  Grande  a  distance  of  about  twelve  miles  north-north- 
west into  Brewster  County.  It  is  some  five  miles  wide  at 
the  river,  and  tapers  to  a  fairly  sharp  point  at  the  north. 
The  crest  of  the  fold  at  the  river  consists  of  the  Edwards 
limestone.  This  formation  at  the  north  dips  under  the 
Del  Rio  horizon  which  is  represented  by  only  a  few  feet  of 
marly  material  in  turn  dipping  under  the  Buda  limestone. 
The  northernmost  end  of  the  ridge  is  covered  by  the  Eagle 
Ford,  or  the  Boquillas  flags,  which  has  a  thickness  in  this 
region  of  about  600  feet.  These  in  turn  go  under  the  Austin 
chalk  and  the  Taylor  marls.  The  softness  of  the  latter  beds 
has  caused  them  to  be  eroded  away  leaving  the  axial  part 
of  the  anticline  as  a  prominent  ridge.  It  should  be  noted 
that  two  sills  of  acidic  volcanic  rock,  one  of  which  is  quite 
glassy,  have  been  intruded  between  some  of  the  layers  of 
the  Eagle  Ford  beds,  and  lie  folded  with  these- 

The  "nose/*  as  petroleum  geologists  would  call  it,  of  the 
plunging  anticline,  is  itself  flexed  downward  at  the  north 
end,  and  this  has  resulted  in  some  transverse  fissuring  of 
the  ridge.  In  these  fissures  the  best  ore  was  first  found, 
and  in  one  of  them  the  principal  ore  so  far  discovered  is 
taken  at  the  present  time.  Most  of  the  work,  both  on  the 


*  Structural  Relations  of  Quicksilver  Deposits.     Mining  World,  Vol. 
34,  May  13,  1911. 


12 


University  of  Texas  Bulletin 


surface  and  underground,  has  been  done  in  the  middle  part 
of  the  Eagle  Ford.  This  part  of  the  Eagle  Ford  consists 
of  calcareous  shaly  flags  that  are  quite  compact  and  fre- 
quently highly  bituminous.  They  have  been  found  to  con- 
tain some  small  pockets  of  oil,  and  small  "seepages"  of  solid 
bituminous  material  have  also  been  noted.  Some  ore  has 


Diagrammatic  section  acrovj  The  Manscal  Anticline  near  The  r\arfh  line  of  section 33 


Diagrammatic  secTion  across  The  Mariscal  AnTiclme  near  the  main  shaft  of  The  Ellis  Mine  in  section 


mmaTic  across The'Manscal  Anticline  abouT  ihree  mile*  south  of  section 33,     "horizontal 
much    -shortened 


Comar%cheon  Limestone     Eacjle  Ford  Shale          AubtmChalK          Tailor  Marl  Igneous  5 


Fig.  3.     Diagrammatic  sections  from  east  to  west  across  the  Mariscal 
Mountain.     Positions  of  sections  shown  in  Fig.  2. 


Anticlinal  Theory  of  Quicksilver  13 

been  found  in  the  contacts  of  the  shales  with  the  igneous 
rocks  and  the  igneous  rock  itself  has  been  found  to  be 
slightly  impregnated  with  cinnabar,  especially  close  to  its 
contacts  with  the  shale.  Judging  by  conditions  in  other 
mines,  richer  deposits  may  be  expected  where  fissures  cut 
the  contact  of  the  Buda  limestone  with  the  overlying  Eagle 
Ford  and  also  at  the  depth  where  they  may  cut  the  Del  Rio 
horizon.  Here  the  underlying  Georgetown  or  Edwards  fur- 
nish a  more  open  body  of  rock  than  either  the  overlying 
Buda  or  the  marly  sheet  representing  the  Del  Rio,  which  is 
quite  thin.  The  workings  in  this  mine  have  not  yet  ex- 
tended down  to  these  deeper  horizons.  The  accompanying 
illustrations  are  intended  to  more  clearly  represent  the  geo- 
logic conditions  at  this  mine. 

The  first  mining  in  the  Terlingua  district  was  done  on 
Sections  40,  41,  58,  and  59,  in  Block  G-12,  in  Brewster 
County.  This  group  of  mines  lies  farthest  to  the  west 
in  the  district.  It  is  at  the  upper  fold  of  a  monocline, 
dipping  south,  at  a  point  where  the  Georgetown,  immedi- 
ately below  the  Del  Rio  clay,  has  been  laid  bare  by  the  re-r 
moval  of  this  clay  and  other  overlying  beds.  The  Del  Ric 
still  underlies  California  Hill,  which  is  capped  by  a  smaller 
remnant  of  the  Buda  limestone.  It  goes  down  under  this 
limestone,  which  caps  the  series  of  hills  to  the  south,  called 
the  Tres  Cuevas  Mountain  and  the  Sierra  del  Cal.  The 
country  north  of  this  line  of  hills  is  traversed  by  several 
small  faults  and  a  larger  one  which  runs  from  west-north- 
west to  east-southeast.  This  fault  has  a  downthrow  to  the 
north,  of  from  a  few  to  perhaps  eighty  feet.  Following 
the  north  side  of  this  fault,  a  remnant  of  the  Del  Rio  clay 
still  is  seen.  The  block  of  rock  south  of  this  fault  dips 
to  the  south  and  in  about  a  half  mile  from  the  fault,  the 
general  dip  to  the  south  rapidly  brings  down  the  suc- 
cession of  Cretaceous  beds  standing  at  angles  of  from  twenty 
to  sixty  degrees.  The  strike  of  this  strong  dip  is  shown 
in  the  Sierra  del  Cal.  The  south  dip  is  continued  by  a  much 
gentler  dip  in  the  same  direction  for  a  distance  of  three 
miles.  To  me  it  is  a  significant  feature  that  the  monocline 
of  the  Sierra  del  Cal  forms  an  arc  of  a  circle,  cr  a  periphery 


14 


University  of  Texas  Bulletin 


of  a  dome,  as  it  were,  and  that  the  ground  under  the  Del 
Rio  clay  in  the  subtended  segment  of  this  dome  has  been 
very  generally  mineralized.  Here  is  a  tract  toward  which 
an  inverted  funnel-like  attitude  of  the  impervious  Del  Rio 
clay  would  naturally  drive  any  ascending  currents  of  min- 
eralizing solutions,  gathered  from  a  large  collecting  area 
to  the  south. 


Fig.  4.      Sketch  map  of  the  vicinity  of  the  Marfa  and  Mariposa  and  the  Old 
Terlingua  Mines  showing  the  position  of  section  given  in  Fig.  5. 

The  ore  .in  the  Marfa  and  Mariposa  mine,  and  also  in  the 
old  Terlingua  mine,  has  been  deposited  mostly  in  joints, 
fissures,  and  cavernous  openings  that  extend  down  in  the 
upper  surface  of  the  Georgetown  limestone.  The  material 
filling  these  fissures  is  locally  known  as  jaboncillo.  It  is 
a  material  of  mixed  nature,  consisting  in  places  largely  of 
clay  and  in  other  places  of  material  quite  like  caliche.  It 
is  evident  that  it  has  been  formed  in  these  fissures  partly 
by  precipitation  from  solutions  which  have  followed  the 
lower  surface  of  the  Del  Rio  clay,  and  partly  also  by  the 
Del  Rio  clay  itself  which  has  settled  perhaps  gradually 


Anticlinal  Theory  of  Quicksilver  15 

into  solution  caverns,  pari  passu  with  their  enlargement 
by  solution.  The  jaboncillo  frequently  contains  fragments 
of  the  limestone  itself  and  is  in  places  not  unlike  a  fault 
breccia,  cemented  with  calcareous  material.  At  the  sur- 
face, this  jaboncillo  in  places  changes  into  caliche,  clearly 
formed  at  a  recent  date.  Even  this  caliche  contains  frag- 
ments of  cinnabar,  which  apparently  have  been  entombed 
quite  recently  in  the  formation  of  superficial  caliche.  The 
fissures  extend  to  varying  depths  and  no  doubt  in  some 
place,  or  places,  join  "pipes,"  most  probably  along  fault 
lines,  through  which  the  quicksilver  has  ascended.  The  great 
number  of  these  ore  bodies  gives  out  in  less  than  twenty 
feet  below  the  upper  surface  of  the  Georgetown  limestone 
and  the  mining  on  most  of  the  hills  in  the  sections  men- 
tioned has  been  done  in  shallow  pits  which  stud  the  land  at 
the  present  time.  In  places,  ore  occurs  at  greater  depths 
and  some  fissures  have  been  found  with  open  vugs  set  with 
calcite  and  gypsum  crystals. 


£agi«,  ford  Buda  Del  Rio  Edwards 


Fig.  5.     Diagrammatic  section  extending  from  south-southwest  to  north- 
northeast  along  the  line  indicated  in  Fig.  4.     Known  occurrences  of 
ore-bearing  rock  are  approximately  indicated  by  solid  black 

It  would  seem  altogether  likely  that  further  prospecting 
may  result  in  the  finding  of  deeper  pipes  and  it  would 
seem  quite  probable  that  these  should,  as  already  stated,  be, 
found  in  some  of  the  faulted  fissures  known  to  exist. 


16  University  of  Texas  Bulletin 

Study  Butte 

The  Study  Butte  is  one  of  the  places  where  extensive 
explorations  have  been  made.  It  is  the  site  of  two  quick- 
silver mines  which  are  both  in  operation  at  the  present 
time:  the  Big  Bend  mine  and  the  Texas  Almaden  mine, 
These  are  adjoining  properties,  the  Texas  Almaden  lying 
on  the  north  of  the  Butte,  with  the  Big  Bend  on  the  south. 
They  are  at  present  operated  jointly.  Study  Butte  rises 
from  an  area  of  the  Taylor  marl  which  is  exposed  over  most 
of  Section  216,  east  of  the  center  of  which  the  Butte  is 
located.  The  Butte  is  clearly  the  result  of  differential  ero- 
sion- It  rises  from  a  low  "flat,"  where  erosion  has  for  some 
time  been  in  active  progress  on  the  soft  marls  overlying  the 
Austin  chalk.  The  Butte  itself  consists  of  a  dike-like  body 
of  andesite,  locally  known  as  rhyolite,  which  has  cut  the 
marls,  now  mostly  eroded,  at  an  angle  of  about  45  degrees, 
hading  to  the  north.  The  dike  itself  has  been  penetrated 
by  a  mining  tunnel  from  north  to  south,  and  along  this 
tunnel  the  intrusive  measures  about  300  feet.  Both  its 
contacts  with  the  marls  show  clearly  in  the  drift.  The 
igneous  rock  has  been  considerably  altered  on  the  north 
side,  some  of  it  having  been  kaolinized.  This  oblique  dike, 
if  I  may  call  it  such,  forms  the  crest  of  the  Butte.  Under- 
lying it  on  the  south  side  are  the  upper  parts  of  the  Taylor 
marls,  and  these  have  been  sapped  away,  causing  the  dike 
to  rise  in  a  vertical  cliff  above  it.  On  the  north  side  the 
beds  exposed  belong,  as  I  judge,  to  the  upper  part  of  the 
Austin  chalk  horizon,  and  have  been  slightly  indurated.  This 
has  caused  a  part  of  them  to  be  left  as  a  cover  over  the 
igneous  rock  on  the  slope  of  the  dike.  At  the  foot  of  the 
hill  on  the  north  side,  the  intruded  sediments  are  about 
100  feet  deep.  Explorations  which  have  been  made  by  the 
diamond  drill  show  that  at  this  point  the  intrusive  measures 
400  feet  in  thickness  and  is  underlaid  by  a  dark  clay.  This 
part  of  the  intrusion  is  of  the  nature  of  a  laccolith.  At  a 
depth  of  500  feet  it  rests  on  a  black  marl  or  clay,  possibly 
corresponding  to  the  Austin  chalk.  The  remaining  over- 
lying beds  on  the  north  side  lie  nearly  flat  or  show  even  a 


Anticlinal  Theory  of  Quicksilver 


17 


slight  dip  to  the  south.     On  the  south  side  of  the  intru- 
sive the  Taylor  marls  seem  to  lie  perfectly  horizontal. 


!      ') 


v  uyy 

,    \    l;p 

>       <  V          ,>.<" 


yy  /  ' '^- 

„-*-''      /' 


Fig.  6. 


Sketch  map  of  Study  Butte  showing  the  position 
of  the  section  in  Fig.  7. 


This  mine  is  known  for  its  quantity  of  low  grade  ore 
which  has  been  found  on  the  north  side  of  the  dike.  Some 
high  grade  ore  has  been  found  in  the  contact  between  the 
overlying  marls  and  the  dike,  and  also  in  some  fissure  veins 
that  cut  the  andesite.  These  stand  almost  vertically,  ap 
parently  through  the  entire  depth  of  the  intrusive,  on  the 
Texas  Almaden  property.  While  the  geological  condition? 
in  this  mine  thus  differ  radically  in  some  respects  frofti 
those  in  the  other  mines  of  the  district,  one  can  readily 
see  the  similarity  of  the  conditions  in  both.  The  quicksilver 
has  evidently  followed  the  fissures  in  the  intrusive  up  tc 
its  upper  contact  with  the  overlying  shales-  From  here  it 


18 


University  of  Texas  Bulletin 


has  partly  followed  continuations  of  the  same  fissures  up- 
ward in  the  shale,  while  much  of  it  has  also  been  diverted 
by  the  close  texture  of  these  shales  and  has  followed  th^« 
slanting  contact  on  the  upper  surface  of  the  intrusive.  Con- 
siderable ore  has  been  recovered  from  this  contact  up  to 
the  top  of  the  Butte  and  along  this  path  the  intrusive  itself 
has  yielded  values  in  several  places.  The  lower,  or  south 
contact  of  the  intrusive,  on  the  other  hand,  has  so  far 
yielded  no  ore. 


Toy!  or    Marl 


Intrusive 


Veins 


Fig.  7.     Section  of  Study  Butte  extending  from  north  to  south  along  the 

line  indicated  in  Fig.  6.     Known  occurrences  of  cinnabar-bearing 

rock  roughly  indicated  by  solid  black. 

The  Christmas  Mountain  Prospects 

One  of  the  most  interesting  occurrences  of  quicksilver  in 
this  region,  from  a  theoretical,  not  to  say  didactic,  stand 
point,  is  to  be  found  in  the  Christmas  Mountain.  This 
mountain  forms  one  of  the  most  perfect  dome  structures  in 
the  region.  It  is  located  about  twelve  miles  northwest  of 
the  Chisos  Mountains.  The  dome  has  a  length  from  north- 
northwest  to  south-southeast  of  about  five  miles  and  a  width 
of  about  four  miles.  The  greatest  vertical  displacement  at 
the  center  of  this  dome  is  at  the  least  4000  feet.  The  moun- 


Anticlinal  Theory  of  Quicksilver 


19 


tain  itself  rises  above  the  surrounding  country  to  a  height 
of  2000  feet.  It  consists  of  the  Edwards  limestone  which 
evidently  has  been  lifted  up  by  a  laccolitic  intrusion.  At 
the  foot  of  the  mountain  we  find  the  Upper  Cretaceous  dip- 
ping outward  in  all  directions  away  from  the  uplift  at  high 
angles.  It  is  evident  that  the  Upper  Cretaceous  covered 
the  entire  uplift  at  one  time,  and  has  been  removed  by 
erosion  from  its  central  area.  The  uplift  is  somewhat  un- 
symmetrical.  The  slope  to  the  east  and  northeast  is  more 
gentle  than  that  to  the  west  and  southwest.  In  fact,  the 
west  side  of  the  mountain  is  at  some  points  much  sheared 
and  faulted.  At  the  south  end  the  limestones  are  fissured 
and  the  fissures  are  filled  with  calcite. 


4  Miles 


Fig.   8.     Map  of   Christmas  Mountain   showing  approxi- 
mate position  of  section  in  Fig.  9. 


20  University  of  Texas  Bulletin 

On  the  highest  point  of  this  mountain  there  is  a  small 
cinnabar  prospect.  The  ore  occurs  in  what  appears  to  b° 
cavernous  places  in  the  Edwards  limestone.  This  evidently 
is  part  of  the  mineralization  which  once  existed  under  tf 
Del  Rio  clay,  that  immediately  overlay  the  limestone  before 
it  was  removed  by  erosion-  Other  small  prospects  have 
been  reported  from  fissures  on  the  west  side  of  the  moun- 
tain. At  the  foot  of  the  mountain  where  the  Buda  and 
the  Del  Rio  clay  set  in,  cinnabar  has  been  found  in  fissures 
and  cavernous  places  at  several  points.  Float  has  been 
reported  also  from  the  east  slope  of  the  mountain.  It  should 
be  recalled  that  the  Christmas  Mountain  lies  in  the  peri- 
phery of  the  quicksilver  district,  but  it  seems  that  the  size 
and  perfection  of  this  structure  has  resulted  even  here  in 
the  accumulation  of  some  of  the  mineralizing  solutions. 


De\R»o. 


Fig.    8.     Diagrammatic   section  from   southwest  to   northeast   across   the 
Christmas  Mountain  along  line  indicated  in  Fig.  8.     Known  cin- 
nabar-occurrences indicated  by  solid  black. 

The  Workings  on  Section  248 

On  Section  248,  in  Block  G-4,  considerable  prospecting 
has  been  done,  though  at  present  no  mining  is  carried  on 
there.  The  country  which  has  been  prospected  on  this  sec- 
tion is  covered  by  a  relatively  quite  stony  layer  of  the  Austin 
chalk,  from  which  a  less  resistant  and  more  argillaceous 
part  of  this  formation  has  recently  been  eroded  away.  We 
have  therefore  here  the  same  conditions  as  have  been  noted 


Anticlinal  Theory  of  Quicksilver 


21 


I  Mile. 


Fig.  10.     Map  of  Section  248,  and  parts  of  sections  247, 
230,  and  299,  Block  G-4,  west  of  the  Chisos  Moun- 
tains, Brewster  County,  showing  location  of 
section  a-b,  in  Fig.  11. 

at  other  places.  The  mercurial  solutions  in  passing  upward 
have  been  deflected  on  the  upper  contact  of  the  at  present 
exposed,  more  open  layer,  with  the  less  pervious  and  more 
clayey  layer  that  overlay  it  at  the  time  the  ore  was  de- 
posited. The  prospecting  has  been  done  on  the  west  limb 
of  a  low  anticline,  which  runs  a  course  from  north  to  south 
and  whose  axis  is  not  far  from  the  east  line  of  the  section 
The  east  limb  of  this  anticline  has  a  higher  dip  than  the 
west  limb.  The  ore  has  been  found  along  some  fissures 
that  run  vertically  to  this  axis  and  it  has  been  followed 
downward  for  apparently  half  a  hundred  feet  or  more.  At 
several  places  in  one  of  the  worked  joints,  the  country  rock 
has  been  found  to  be  highly  charged  with  black  bitumen 


22 


University  of  Texas  Bulletin 


Fig.   11.     Diagrammatic  section  along  the  line  a-b  in  Fig.    10.     Known 
occurrences  of  cinnabar  are  indicated  by  solid  black. 

The  Coltrin's  Camp  Prospect 

Coltrin's  Camp  is  a  prospect  where  a  little  work  was  done 
in  the  early  days  of  mining  in  this  district.  It  is  situated 
under  the  south  escarpment  of  the  Reed  Plateau.  The 
structural  conditions  are  somewhat  similar  to  those  on  the 
Marfa  and  Mariposa  property.  The  plateau  is  an  uplifted 
part  of  the  Edwards  limestone,  which  rises  in  a  small  table 
land  less  than  one  mile  wide  and  not  quite  four  miles  in 
length.  On  the  south  side  of  this  plateau  the  Cretaceous 
series  are  abruptly  turned  down,  and  the  softer  formations 
lying  above  the  Edwards  are  mostly  eroded  away.  The 
cinnabar  here  occurs  in  some  veins  vertical  to  the  longer 
axis  of  the  plateau,  which  cut  the  Eagle  Ford  formation. 
Here  as  elsewhere,  therefore,  it  would  seem  that  precipita- 
tion of  ore  has  taken  place  at  a  point  where  the  mineralized 
solutions,  following  fissures,  encountered  relatively  imper- 
vious materials  after  traversing  the  more  easily  penetrable 
limestones  of  Comanchean  age,  which  in  this  region  measure 
more  than  1500  feet  in  thickness.  (See  Figures  14  and  16.) 

The  Laguna  Float 

Cinnabar  has  also  been  reported  from  the  top  of  the 
Chisos  Mountains.  The  Eagle  Ford  underlies  the  country 
surrounding  the  central  part  of  the  Chisos  Mountains  at  a 
depth  not  far  from  sea  level.  In  the  region  west  of  a  point 


Anticlinal  Theory  of  Quicksilver 


23 


.  X     t>.  f  —  joS  i 


known  as  Laguna,  on  top  of  the  Chisos  Mountains,  this 
formation  lies  at  an  elevation  of  7000  feet  above  sea  level, 
dipping  with  a  high  angle  to  the  northeast.  It  is  not  known 
to  the  writer  where  the  float  reported  was  found,  or  in 
association  with  what  rocks  in  the  upper  part  of  the  moun 
tains  it  occurred.  The  Chisos  Mountain  structure  may 


24 


University  of  Texas  Bulletin 


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Anticlinal  Theory  of  Quicksilver  25 

perhaps  be  called  a  greatly  faulted  anticline.  This  anti- 
cline is  not  very  extensive  laterally,  but  it  is  unusually 
high.  No  doubt  it  is  partly  filled  with  intrusives  which, 
where  exposed,  appear  to  be  of  fine  compact  texture.  Pre- 
sumably the  mineralizing  currents  depositing  cinnabar  near 
Laguna  have  here  followed  some  more  open  contact. 

The  Chisos  Mine 

The  Chisos  Mining  Company  property  illustrates,  it  seems 
to  me,  the  general  conditions  of  the  deposition  of  the  quick- 
silver ore  in  this  region  perhaps  better  than  any  other  mine 
in  the  district.  It  so  happens  that  the  present  writer  has 
had  his  best  opportunities  for  observation  on  this  property. 
This  is  located  on  the  north  side  of  a  long  and  narrow  rift 
valley  known  as  the  Long  Draw.  This  valley  has  a  width 
of  from  one  to  two  miles  and  a  length  of  nearly  nine  miles 
The  structure  which  has  caused  this  valley  is  continuous 
for  this  distance.  The  valley  is  caused  by  the  sinking  of  a 
narrow  strip  of  the  Cretaceous  formations  of  the  dimensions 
indicated,  and  runs  a  curving  course  following  roughly  th< 
periphery  of  a  circle  some  nine  miles  in  diameter,  with  its 
center  from  the  mine  northeast.  The  vertical  displacement 
of  this  sunken  slice  of  the  formations  amounts  to  from  1500 
to  2000  feet  with  reference  to  the  adjacent  block  on  the 
south.  With  reference  to  the  adjacent  block  on  the  north, 
the  displacement  ranges  from  500  to  1000  feet.  At  the 
Chisos  Mining  Company's  Mine,  two  parallel  anticlines  fol- 
low the  sunken  elongated  block  of  the  valley,  running  from 
west-northwest  to  east-southeast.  Both  of  these  anticlines 
plunge  to  the  southeast.  The  mine  is  located  at  a  point 
where  this  plunging  is  steepest  and  where  it  has  caused  a 
transverse  fracturing  of  the  south  anticline.  The  plunging 
here  brings  the  Eagle  Ford  below  the  surface.  The  south 
side  of  this  anticline  coincides  more  or  less  with  the  fissures 
separating  the  sunken  block  of  the  valley  from  the  block  to 
the  north,  which  is  folded  into  the  two  anticlines  referred 
to. 


26 


University  of  Texas  Bulletin 


On  both  sides  of  the  Long  Draw  can  be  seen  lagging 
blocks  such  as  usually  appear  along  faults  of  considerable 
movement.  The  faults  which  separate  the  long  curving 
sunken  block  from  the  ground  on  either  side  are  thus  both 
compound  faults.  These  lagging  blocks  nearly  always  dip 
in  the  direction  of  the  drag.  In  the  ground  where  most  of 
the  quicksilver  has  been  taken  in  this  mine,  occurs  one  of 
these  lagging  blocks.  It  is  cut  off  to  the  west  by  an  oblique 


s-S 


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Anticlinal  Theory  of  Quicksilver 


27 


fault,  which  crosses  the  north  fault  of  the  rift  valley  and 
extends  from  the  north-northwest  to  the  south-southeast 
Most  of  the  movement  in  the  compound  fault  has  taken  place 
between  the  south  side  of  the  lagging  block  and  the  sunken 
rift  block.  The  displacement  here  amounts  to  probably 
1000  feet  while  the  displacement  between  the  lagging  block 
and  the  main  block  on  the  north  is  less  than  40  feet.  Ever 
this  small  movement  is  evidently  in  places  taken  up  by  steep 
dips.  Ore  has  been  found  in  all  three  of  the  faults  de- 
scribed and  it  has  also  been  found  in  fissures  accompanied 
by  faulting  across  the  most  rapidly  plunging  part  of  the 
anticline. 


BlocK 


C    ^T±-r±-T±rTm 


Tayl 


AubtinChfllK. 


Buda.  Del  Rio.       Georgetown.   Edwards. 


Fig.    15.     Diagrammatic    section   running    approximately   from    north    to 

south  along  the  line  a-b  indicated  in  Fig.   14.     Known  localities 

of  ore-bearing  rock  indicated  by  solid  black. 

Some  early  work  on  this  property  was  done  at  the  south- 
west corner  of  the  lagging  block  in  the  fissure  separating 
the  rift  block  from  the  southwest  corner  of  the  lagging  block 
referred  to.  This  deposit  did  not  last  long.  It  was  on  what 


28  University  of  Texas  Bulletin 

was  formerly  known  as  the  McKinney-Parker  property.  The 
first  work  in  the  Chisos  mine  proper  was  done  in  the  Grace 
Canyon,  in  fissures  that  cut  the  plunging  anticline  tran- 
versely  to  its  axis  where  its  plunge  is  steepest.  At  this 
point  the  Eagle  Ford  shales  measure  some  540  feet  in  thick 
ness.  For  the  first  years  mining  here  did  not  go  below  this 
depth.  The  ore  showed  variations  according  to  the  nature 
of  the  beds  in  which  the  veins  occur,  but  it  was  not  until  the 
contact  between  the  Eagle  Ford  with  the  underlying  Buda 
limestone  was  reached,  that  the  ore  began  to  improve. 
Within  the  last  two  years  some  entries  have  gone  below 
the  Del  Rio  clay  and  some  fine  ore  has  been  worked  in  the 
top  of  the  Georgetown  limestone.  This  is  north  of  the  fis- 
sure marking  off  the  lagging  block  from  the  main  block 
on  the  south  side  of  the  southernmost  of  the  two  parallel 
anticlines.  Most  of  the  workings  in  the  mine  are  along 
this  fissure,  and  most  of  the  ore  has  so  far  been  taken  out 
from  this  fissure,  and  from  the  ground  immediately  to  the 
south  and  east  of  it.  The  ore  may  be  characterized  as  oc- 
curring in  irregular  bodies  along  this  fissure.  There  has 
also  been  mined  much  low  grade  ore  in  the  bituminous  flags 
and  marls  close  to  these  fissures.  Calcite  forms  the  prin 
cipal  gangue  in  the  fissures.  Looking  at  the  distribution 
of  ore  in  the  entries  which  follow  the  principal  fissure,  i* 
appears  that  the  productive  ground  is  more  scattered  on  the 
upper  levels  than  below,  where  it  contracts  to  a  limited  parfe 
of  the  fissure,  like  a  pipe,  between  Shafts  Numbers  3  and  8 
at  the  southeast.  Examining  the  ground  explored  in  thr 
lower  entries  it  is  found  that  the  sharpest  plunge  of  the  an 
ticline  is  crossed  at  this  point  by  a  zone  of  what  may  be 
called  a  fault  breccia,  consisting  mostly  of  large  blocks  and 
smaller  fragments  of  Buda  limestone  scattered  through  a 
large  matrix  of  broken  and  fractured  Del  Rio  clay.  This 
pipe  has  perhaps  been  one  of  the  principal  points  where  the 
mineralizing  solutions  ascended  to  the  upper  levels.  It 
would  seem  natural  that  they  should  scatter  on  reaching 
the  thin  bedded  flags  of  the  Eagle  Ford.  From  this  cir- 
cumstance it  would  be  natural  to  expect  that  some  solu- 
tions traversed  the  ground  to  the  north  under  the  Del  Rio 


Anticlinal  Theory  of  Quicksilver 


29 


clay,  away  from  the  same  fissures  and  their  principal 
zone  of  deposition  can  be  expected  here,  as  at  other  places, 
at  the  Del  Rio-Georgetown  contact,  owing  to  the  impervious 
nature  of  the  Del  Rio  clay. 


30  University  of  Texas  Bulletin 

CONCLUSIONS 

From  the  foregoing  accounts  of  practically  all  the  im 
portant  occurrences  in  the  Terlingua  district,  it  appears  to 
me  that  the  relation  of  the  ore  to  structures  suitable  for 
collecting  and  impeding  the  progress  of  ascending  vaporous 
or  other  solvents  in  mineralizing  solutions  can  hardly  be  a 
mere  coincidence.  There  is,  however,  one  decided  differ- 
ence in  the  anticlinal  distribution  of  these  cinnabar  ores* 
from  the  anticlinal  accumulation  of  the  bitumens.  It  is  evi- 
dent that  in  its  ascension  from  great  depths,  mercurial 
fumes  in  solution  would  be  more  or  less  limited  to  per- 
vious conduits  furnished  by  the  rocks  traversed.  They  have 
followed  joints,  fissures,  fault  planes,  and  contacts.  They 
have  clearly  also  relatively  freely  invaded  any  pervious 
strata  coming  in  their  way.  In  the  rocks  which  are  most 
impervious,  the  quicksilver  ore  is  confined  to  joint  cracks 
and  occurs  as  "paint"  in  thin  films  on  the  outside  of  the 
blocks  which  these  cracks  separate.  Elsewhere  the  ore  oc- 
curs in  fissure  veins  measuring  from  a  small  fraction  of  an 
inch  to  several  feet  in  thickness.  It  also  occurs  along  con- 
tact planes  of  igneous  bodies,  which  traverse  compact  shales 
and  marls. 

In  general,  it  can  be  said  that  the  greatest  amount  of  ore 
has  so  far  been  found  in  fissured  and  faulted  parts  of  anti- 
clines, domes,  and  arrested  monoclines.  A  recognition  of 
the  significance  of  these  facts  enabled  the  writer  many  years 
ago  to  predict  the  depth  and  place  at  which  the  richest  ores 
in  the  Chisos  mine  would  be  likely  to  be  found.  This  pre- 
diction has  in  so  far  proved  satisfactory  that  it  encouraged 
the  management  to  go  through  several  hundred  feet  of 
somewhat  unprofitable  ground,  and  that  this  work  enabled 
it  to  reap  a  satisfactory  reward  after  some  years  of 
more  or  less  comparatively  unprofitable  exploration  of  the 
upper  levels  in  the  mine. 


T. 


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